The distance and neutral environment of the massive stellar cluster Westerlund 1

The goal of this study is to determine a distance to Westerlund 1 independent of the characteristics of the stellar population and to study its neutral environment, using observations of atomic hydrogen. The HI observations are taken from the Southern Galactic Plane Survey to study HI absorption in the direction of the HII region created by the members of Westerlund 1 and to investigate its environment as observed in the HI line emission. A Galactic rotation curve was derived using the recently revised values for the Galactic centre distance of $R_\odot = 7.6$ kpc, and the velocity of the Sun around the Galactic centre of $\Theta_\odot = 214$ km s$^{-1}$. The newly determined rotation model leads us to derive a distance of $3.9\pm 0.7$ kpc to Westerlund 1, consistent with a location in the Scutum-Crux Arm. Included in this estimate is a very careful investigation of possible sources of error for the Galactic rotation curve. We also report on small expanding HI features around the cluster with a maximum dynamic age of 600,000 years and a larger bubble which has a minimum dynamic age of 2.5 million years. Additionally we re-calculated the kinematic distances to nearby HII regions and supernova remnants based on our new Galaxic rotation curve. We propose that in the early stages of the development of Wd 1 a large interstellar bubble of diameter about 50 pc was created by the cluster members. This bubble has a dynamic age similar to the age of the cluster. Small expanding bubbles, with dynamical ages $\sim 0.6$ Myr are found around Wd 1, which we suggest consist of recombined material lost by cluster members through their winds.Comment: 8 pages, accepted for publication in A&

Schwarzschild horizon and the gravitational redshift formula

The gravitational redshift formula is usually derived in the geometric optics approximation. In this note we consider an exact formulation of the problem in the Schwarzschild space-time, with the intention to clarify under what conditions this redshift law is valid. It is shown that in the case of shocks the radial component of the Poynting vector can scale according to the redshift formula, under a suitable condition. If that condition is not satisfied, then the effect of the backscattering can lead to significant modifications. The obtained results imply that the energy flux of the short wavelength radiation obeys the standard gravitational redshift formula while the energy flux of long waves can scale differently, with redshifts being dependent on the frequency.Comment: Revtex, 5 p. Rewritten Sec. II, minor changes in Secs III - VII. To appear in the Classical and Quantum Gravit

Optical scalars in spherical spacetimes

Consider a spherically symmetric spacelike slice through a spherically symmetric spacetime. One can derive a universal bound for the optical scalars on any such slice. The only requirement is that the matter sources satisfy the dominant energy condition and that the slice be asymptotically flat and regular at the origin. This bound can be used to derive new conditions for the formation of apparent horizons. The bounds hold even when the matter has a distribution on a shell or blows up at the origin so as to give a conical singularity

Diffusion of the scalar field energy due to the backscattering off Schwarzschild geometry

This note tackles the problem of the backscattering of a mass-less scalar field in the case of Schwarzschildean space-time. It shows that the effect depends both on a distance from the horizon and on the wave length. The obtained estimates significantly improve former results.Comment: LaTeX 2e, 6 page

Can Schwarzschildean gravitational fields suppress gravitational waves?

Gravitational waves in the linear approximation propagate in the Schwarzschild spacetime similarly as electromagnetic waves. A fraction of the radiation scatters off the curvature of the geometry. The energy of the backscattered part of an initially outgoing pulse of the quadrupole gravitational radiation is estimated by compact formulas depending on the initial energy, the Schwarzschild radius, and the location and width of the pulse. The backscatter becomes negligible in the short wavelength regime.Comment: 18 pages, Revtex. Added three references; a new comment in Sec. 7; several misprints corrected. To appear in the Phys. Rev.

Transport in Graphene Tunnel Junctions

We present a technique to fabricate tunnel junctions between graphene and Al and Cu, with a Si back gate, as well as a simple theory of tunneling between a metal and graphene. We map the differential conductance of our junctions versus probe and back gate voltage, and observe fluctuations in the conductance that are directly related to the graphene density of states. The conventional strong-suppression of the conductance at the graphene Dirac point can not be clearly demonstrated, but a more robust signature of the Dirac point is found: the inflection in the conductance map caused by the electrostatic gating of graphene by the tunnel probe. We present numerical simulations of our conductance maps, confirming the measurement results. In addition, Al causes strong n-doping of graphene, Cu causes a moderate p-doping, and in high resistance junctions, phonon resonances are observed, as in STM studies.Comment: 22 pages, 5 figure

Three-dimensional shapelets and an automated classification scheme for dark matter haloes

We extend the two-dimensional Cartesian shapelet formalism to d-dimensions. Concentrating on the three-dimensional case, we derive shapelet-based equations for the mass, centroid, root-mean-square radius, and components of the quadrupole moment and moment of inertia tensors. Using cosmological N-body simulations as an application domain, we show that three-dimensional shapelets can be used to replicate the complex sub-structure of dark matter halos and demonstrate the basis of an automated classification scheme for halo shapes. We investigate the shapelet decomposition process from an algorithmic viewpoint, and consider opportunities for accelerating the computation of shapelet-based representations using graphics processing units (GPUs).Comment: 19 pages, 11 figures, accepted for publication in MNRA

The Jang equation, apparent horizons, and the Penrose inequality

The Jang equation in the spherically symmetric case reduces to a first order equation. This permits an easy analysis of the role apparent horizons play in the (non)existence of solutions. We demonstrate that the proposed derivation of the Penrose inequality based on the Jang equation cannot work in the spherically symmetric case. Thus it is fruitless to apply this method, as it stands, to the general case. We show also that those analytic criteria for the formation of horizons that are based on the use of the Jang equation are of limited validity for the proof of the trapped surface conjecture.Comment: minor misprints correcte

Sufficient Conditions for Apparent Horizons in Spherically Symmetric Initial Data

We establish sufficient conditions for the appearance of apparent horizons in spherically symmetric initial data when spacetime is foliated extrinsically. Let $M$ and $P$ be respectively the total material energy and the total material current contained in some ball of radius $\ell$. Suppose that the dominant energy condition is satisfied. We show that if $M- P \ge \ell$ then the region must possess a future apparent horizon for some non -trivial closed subset of such gauges. The same inequality holds on a larger subset of gauges but with a larger constant of proportionality which depends weakly on the gauge. This work extends substantially both our joint work on moment of time symmetry initial data as well as the work of Bizon, Malec and \'O Murchadha on a maximal slice.Comment: 16 pages, revtex, to appear in Phys. Rev.

Features of gravitational waves in higher dimensions

There are several fundamental differences between four-dimensional and higher-dimensional gravitational waves, namely in the so called braneworld set-up. One of them is their asymptotic behavior within the Cauchy problem. This study is connected with the so called Hadamard problem, which aims at the question of Huygens principle validity. We investigate the effect of braneworld scenarios on the character of propagation of gravitational waves on FRW background.Comment: to appear in ERE09 proceeding